The sesame contains about 2% of oxalic acid (including an oxalic acid base such as one existing as a metal salt) most of which is present in its testa. The calcium of sesame exists as calcium oxalate in its testa. The oxalic acid contained in foods binds to calcium, adversely affecting the ratio of absorption in the living body (hereinafter referred to as "internal absorption ratio"). A number of researches were reported on the internal absorption ratio of calcium oxalate in spinach. The internal absorption ratio of calcium in spinach is reportedly in the range of approximately from 1/5 to 1/10 the internal absorption ratio of calcium in cabbage with a low content of oxalic acid.
The sesame is a food with a high content of phytic acid (including a phytic acid base such as one existing as a metal salt). Generally phytic acid is predominantly contained in grains, beans, seeds or the like. Reportedly phytic acid relatively firmly binds to mineral components such as calcium, magnesium, iron, zinc or the like, impairing the internal absorption ratio of minerals in grains, beans, seeds or the like. According to numerous reports, when a large amount of phytic acid is ingested, the mineral simultaneously taken in is almost excreted as adsorbed on the phytic acid and scarcely left.
Despite the great popularity of calcium in recent years, attention has not been drawn to the calcium contained in the testae of sesame presumably for the above-mentioned reason. In such situation, the present inventors made attempts to recover free calcium (calcium not bonded to oxalic acid or phytic acid) from the testae of sesame and found it difficult to simultaneously conduct a first step of decomposing calcium oxalate to give free calcium and a second step of removing the phytic acid bonded to mineral.